Iron metal reacts vigorously with dilute nitric acid or concentrated nitric acid to give ferrous nitrate [Fe(NO3)2] as the main product along with the evolution of nitrogen dioxide gas (brown fumes).

$$4Fe(s) + 10HNO_3(dil.) \rightarrow $$ $$4Fe(NO_3)_2(aq) + NH_4NO_3(aq) + 3NO_2(g) + 5H_2O(l)$$ $$Fe(s) + 4HNO_{3(conc.)} \rightarrow Fe(NO_3)_3(aq) + NO_2(g) + 2H_2O(g)$$